Step-by-step explanation:

1. a² - 25b²

Difference of two squares:

a² - b² = (a - b)(a + b)

a² - 25b² = (a - 5b)(a + 5b)

2. 4x² - 36 = (2x - 6)(2x + 6)

3. 25y² - z² = (5y - z)(5y + z)

5. 2x² - 18 = 2(x² - 9) = 2(x - 3)(x + 3)

I can't understand the equation in number 4.